Using rapid flow microfluorometry (FMF) for analysis and sorting of cells, aspects of the following projects have been supported during the previous year: (1) study of immunoglobulin gene rearrangement and B-cell maturation, (2) analysis of the relationship between immune response genes and Ia cell-surface determinants, (3) analysis of cell:cell conjugation, (4) detection of anti I-E antibodies, and (5) analysis of idiotypes shared by mouse anti H-2 antibodies, antigens shared by I-E molecules from different species, and antigens shared by mouse MHC products.